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Old 06-27-2010, 07:15 AM #17
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Default Re: How can I understand Light as a Wave?

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Originally Posted by ossumguywill View Post
ahh! There we go. Now I get it (at least with sound and stuff). But what happens with laser light? If the two beams intersect in space, they cancel each other out and the energy is not absorbed by anything... Additionally, if you took two green laser pointers and put the beams together, would they not interfere (constructively or destructively) and change their frequency (as frequency is linked to amplitude)? Wouldn't you end up with different color light?
Last part first, changing amplitude doesn't change frequency or wavelength. Interference won't change frequency or wavelength.

And that is talking about light. If you build some sort of optical experiment that results in interference, energy is still conserved. It is found elsewhere in space, or possibly absorbed by the original, or another, source.


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Old 06-27-2010, 07:20 AM #18
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Default Re: How can I understand Light as a Wave?

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Last part first, changing amplitude doesn't change frequency or wavelength. Interference won't change frequency or wavelength.

And that is talking about light. If you build some sort of optical experiment that results in interference, energy is still conserved. It is found elsewhere in space, or possibly absorbed by the original, or another, source.
Wait a minute... I thought the golden rule with photons was that the frequency is absolutely 100% stuck to the amplitude. Lambda=h/mv, correct? If this applies to light, and interference changes mv (or the equivalent for massless particles lol) then the interference should change lambda as well... if you decrease the energy of a photon, the frequency decreases as well, correct? E=hc/lambda, if E changes both h and c are constants so all that's left to change is lambda.

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Old 06-27-2010, 07:27 AM #19
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Default Re: How can I understand Light as a Wave?

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Wait a minute... I thought the golden rule with photons was that the frequency is absolutely 100% stuck to the amplitude. Lambda=h/mv, correct? If this applies to light, and interference changes mv (or the equivalent for massless particles lol) then the interference should change lambda as well... if you decrease the energy of a photon, the frequency decreases as well, correct? E=hc/lambda, if E changes both h and c are constants so all that's left to change is lambda.
Lambda is wavelength, not amplitude. Frequency is stuck to wavelength and photon energy, not amplitude. Amplitude is not tied to either of those, amplitude changes with the intensity of the light.
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Old 06-27-2010, 08:07 AM #20
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Default Re: How can I understand Light as a Wave?

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Lambda is wavelength, not amplitude. Frequency is stuck to wavelength and photon energy, not amplitude. Amplitude is not tied to either of those, amplitude changes with the intensity of the light.
In other words, amplitude (or the intensity of light) is related to the number of photons. Whereas the energy of each photon is related to its associated wavelength. Great Q&A, brings back a lot of memories .
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Old 06-27-2010, 06:44 PM #21
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Default Re: How can I understand Light as a Wave?

I thought amplitude applied to the energy of the individual photon? Considering it's a wave, that seems appropriate... OK then, if the ENERGY of the photon changes during destructive interference, wouldn't the frequency change?
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Old 06-27-2010, 08:11 PM #22
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Default Re: How can I understand Light as a Wave?

You're now dealing with the reality of wave-particle duality . Remember, a photon is not a wave. And, waves do interference, not photons.

Hence, wave-particle duality .

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Old 06-27-2010, 09:36 PM #23
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Default Re: How can I understand Light as a Wave?

If a photon is not a wave, how does it interfere? And it seems to me a photon IS a wave in basically every respect except for that is has a fixed energy level at a given frequency...
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Old 06-28-2010, 06:02 AM #24
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Default Re: How can I understand Light as a Wave?

Ah, if I may interject something here, the particle wave duality is also part of the debated observer effect in quantum mechanics. The photon can act like both depending on how it's observed.

see Double-slit experiment - Wikipedia, the free encyclopedia
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Old 06-28-2010, 06:51 AM #25
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Default Re: How can I understand Light as a Wave?

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Ah, if I may interject something here, the particle wave duality is also part of the debated observer effect in quantum mechanics. The photon can act like both depending on how it's observed.

see Double-slit experiment - Wikipedia, the free encyclopedia
That works even if you only let through one photon at a time, right?

EDIT: some guy on 4chan answered my second question, but just to make sure(cause it's 4chan): If you have 2 perpendicular beams out of phase by 180, where they intersect goes dark but after that they act like nothing happened, right?

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Old 06-28-2010, 04:12 PM #26
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Default Re: How can I understand Light as a Wave?

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That works even if you only let through one photon at a time, right?
Yep. A very interesting point about diffraction, it happens even if you're firing 1 photon at a time. Send 1 photon at a time, spaced far apart in time, and it'll still form the interference pattern. A single photon, in a way, interferes with itself.

As far as all the other stuff, do some more reading. The energy of a photon it tied to wavelength/frequency, NOT amplitude. Each photon carries a certain quanta of energy. The amplitude of the wave is the deviation in the electric/magnetic field at that location as the wave passes by. Adding more photons in-phase with the original increases the change in the electric field, adding photons out of phase decreases the change in the electric field. Think about this: At a specific point in space, as a wave passes by, 2 times per wavelength that passes by, the magnetic and electric fields go to zero for very short amounts of time, which is the same magnetic/electric fields that exist during total destructive interference. The energy isn't disappearing when the electric and magnetic fields go to zero twice per cycle as a photon passes by a fixed location, right?

A photon's energy is fixed, that does not change with interference. Interference results in changes in the electric and magnetic fields at different points in space as a result of photon interactions, and the photon energies never change.

Energy is always conserved, but interference does happen. To really see it, you need to be able to do the math and see where the energy goes for a specific thought experiment. I can tell you as many times as you want that the energy will be somewhere, but until you can do the math to see it, you may very well have trouble believing it. Just like your thoughts about the phase behavior of the magnetic and electric components of an electromagnetic wave: you need Maxwell's equations to prove to you why the 2 components are in-phase and not offset.

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Old 06-28-2010, 06:23 PM #27
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Default Re: How can I understand Light as a Wave?

Cool, thanks! I swear, I feel like a crack addict not knowing this stuff. I need MORE Oh well, I guess I'll have to wait.
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Old 07-03-2010, 11:15 AM #28
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Default Re: How can I understand Light as a Wave?

An oversimplified way to analyze the physical model of a light wave, is that cosmic rays, some of the shortest are passing through the earth because individual atoms have too much space between sub-atomic particles, relative to the wavelength to stop them. While light rays are stopped/absorbed/reflected by your skin. All because of the differences in relative wavelength/amplitude.
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Old 07-13-2010, 06:52 PM #29
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Default Re: How can I understand Light as a Wave?

Calling light either a particle or wave is incorrect. It's neither. However, modeling light mathematically as either a particle or a wave is very useful for many things. Never confuse a model for the thing itself.

Like waves in water, or sound waves, light will demonstrate the phenomenon of interference and diffraction.

One of the first people to posit a wave theory was Christiaan Huygens:
Christiaan Huygens - Wikipedia, the free encyclopedia

How did he decide light must be a wave? Careful observation of obscure things. Like double refraction and observation of the 'airy disk' seen through a telescope

See some of the images of Huygens principle:
Huygens

Issac Newton had a competing 'Corpuscular', theory of light:
Corpuscular theory of light - Wikipedia, the free encyclopedia

Newton's theory is still with us, except 'corpuscular' is now 'photon'.

An example of how weird this all gets is the famous 'double slit' diffraction experiment. You can easily duplicate this with a laser yourself. I really recommend you watch this five minute video to see it in action (> 1.5 million views):
YouTube - Dr Quantum - Double Slit Experiment
Is it a particle or a wave?

For example, in optics, diffraction limits how much you can magnify something. A reflecting telescope has a primary mirror. In order to produce a good image the 'figure' of the mirror should not deviate by more than 1/8 wave from an ideal parabola. 1/8 of what you might ask? 1/8 wave of whatever wavelength of radiation is being observed - usually green/yellow (sodium) light. A pretty small tolerance of error. When the figure is good the telescope is said to be 'diffraction limited', meaning there's no need to polish and figure the optics beyond this point. However, even when the mirror has an ideal figure diffraction means than an in focus star looks something like this:



It's not a perfect disk, rather it's more of a splash, with concentric rings around a bright central disk. When you look at a star, either naked eye or through a telescope, you're not seeing the surface of the star - it's much too far away. Rather you're seeing an optical phenomenon - the 'Airy-disk':
Airy disk - Wikipedia, the free encyclopedia

The larger the telescope's mirror, the smaller the airy-disk becomes, and its resolving power increases.

Another example of a device that uses the wave nature of light to do useful things is an 'interferometer':
Michelson interferometer:
Michelson interferometer - Wikipedia, the free encyclopedia

Laser speckle is yet another example of interference. Shine a laser into a corner of the ceiling and see the 'speckle' or 'snow' all around the dot. You can't focus on it, it's always sharp. The space of the room is filled with an interference pattern.

Finally, here are some images I took shining a red laser off a frosted incandescent bulb. The laser was a few meters away from the bulb, and projected spot a few meters away from the wall. The images are an interference pattern created by the surface of the lightbulb, and represent a sort of map of it's roughness. Interference. A laser magnifying glass.

http://www.fractalfreak.com/wx/interfere1.jpg
http://www.fractalfreak.com/wx/interfere3.jpg
http://www.fractalfreak.com/wx/interfere5.jpg

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Old 07-13-2010, 10:15 PM #30
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Default Re: How can I understand Light as a Wave?

ossumguywill, if you want to 'see' (actually hear) an easy physical example of destructive interference, get two passive speaker boxes (passive = no internal amplifier), wire one correctly and the other one incorrectly (this means, put + on - and - on +, it won't burn it, it will just make it go out of phase, 180 precisely) then place them so the drivers are facing each other and start playing some music at normal hearing level. You tell us what happens

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Old 07-14-2010, 04:04 AM #31
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Default Re: How can I understand Light as a Wave?

I don't have speaker boxes to do that, but my guess is is that depending on where I'm standing the sound will change in intensity. In some places the sound will constructively interfere, correct?
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Old 07-14-2010, 04:59 AM #32
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Default Re: How can I understand Light as a Wave?

Sure. That's how sound suppression technologies work, like noise-cancelling headphones.

Play the 'inverse' of the noise and you've got silence.

These 'active noise control' techniques have a lot of uses, such as quieting a car engine:
Active noise control - Wikipedia, the free encyclopedia
Toyota develops noise-cancelling system for cars - MotorAuthority
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